Communication System

1. A communication system in which a plurality of communication terminal devices perform radio communication via a network configured by a network-side device including one or a plurality of base station devices, the communication system comprising: a plurality of cells configured by said one or the plurality of base station devices and connected to said plurality of communication terminal devices to perform radio communication with said plurality of communication terminal devices, wherein said plurality of cells are grouped into a plurality of groups by said network-side device and said network-side device notifies said plurality of communication terminal devices of information on said plurality of groups, a communication terminal device, in said plurality of communication terminal devices is configured to (a) in an idle state, select a first group or a second group from the plurality of groups and measure cells belonging to the selected first or second group for re-selection, the first group including a cell on which the communication terminal device camps, and the second group being different from the first group, and (b) in a connected state, select a third group or a fourth group from the plurality of groups and measure cells belonging to the selected third or fourth group for handover, the third group including a cell to which the communication terminal device is connected, and the fourth group being different from the third group.

2. The communication system according to claim 1 , wherein said plurality of cells are grouped based on a cell size of each of the plurality of cells, and said communication terminal device is configured to determine, based on a moving speed of the communication terminal device, which of a fifth group and a sixth group among the plurality of groups is to be selected, the fifth group including a cell having a same cell size as the cell on which the communication terminal device camps or to which the communication terminal device is connected, and the sixth group including a cell having a different cell size from the cell on which the communication terminal device camps or to which the communication terminal device is connected.

3. The communication system according to claim wherein said plurality of cells are grouped based on a cell size of each of the plurality of cells, and said communication terminal device is configured to determine a cell size based on a moving speed of the communication terminal device and select a group including a cell having the determined cell size.